At present, European and American countries and regions have requirements on limit values of emissions from gasoline engines, i.e., the sum of THC and NOx in the exhaust from a two-stroke engine less than 50 CC shall not exceed 50 g/kW·h, and the second phase standard of emission limit values in China is also the same as the requirement in this standard. In the last decade, various countries in the world have been researching new technologies of engines to deal with the upgrading emission standard and increasingly stricter requirements on emission limit values in different phases. On the market, there are many hand-held gardening tools, such as chain saws, powered by gasoline engines, most of which are small two-stroke gasoline engines.
A person understanding the engine technology would know that an engine with a low polution emission value in exhaust (low-emission) generally has a reasonable combustion process, a relatively high indicated cycle efficiency, and a reasonable gas exchange process. At present, some of the known small two-stroke gasoline engines use a low-emission cylinder, which generally has a combustion chamber, an exhaust port, a gas inlet and two air inlets arranged opposite to the exhaust port, and scavenging ducts arranged on both sides, the scavenging ducts being of a built-in type, and the entire scavenging ducts being on one side of the cylinder. There are three types of scavenging duct: in the first type, the scavenging duct is in communication with an inner cylinder bore and is open; in the second type, the middle section of the scavenging duct is a closed passage; and the third type is relatively complex, in which the middle section of the scavenging duct is a closed passage, a processed square cover is arranged outside the cylinder body, and the processed cover and the cylinder body together form part of the shape of the scavenging duct. Several cooling fins are provided at the periphery of the inner cylinder bore and the combustion chamber.
In the working process of an engine equipped with a low-emission cylinder, a fuel gas mixture enters a crankcase through a gas inlet, air enters the crankcase through an air inlet, the concentrated gas mixture is diluted with part of the air to form a combustible gas mixture with a proper concentration, and the other part of the air is stored in the scavenging duct. After the gas mixture which has entered the combustion chamber in the last cycle is ignited and exploded to apply work, the air stored in the scavenging duct firstly enters the combustion chamber for scavenging, and then the gas mixture enters the combustion chamber through the scavenging duct for further scavenging. During exhausting, the air which enter the combustion chamber first is spaced between the exhaust gas and the gas mixture, the exhausting is carried out in the order of exhaust gas, air and gas mixture, and the exhaust port is closed at a proper time. Emission values of such low-emission engines are generally 70-100 g/kW·h, the exhausted gas performs secondary combustion under the action of a catalyst in a muffler so as to purify the exhaust gas, and the emission value of the post-processed pollutant in the exhaust is generally decreased to about 30 g/kW·h so as to satisfy the requirements on emission values.
With increasingly stricter requirements on low emission of gasoline engines, the engines which can take the power performance, economical efficiency and emission performance into account are more and more complex, and reducing pollutant emissions by improving the design of the engine itself and optimizing operation parameters is a tendency for the development of engines. The engines in the prior art have a high original emission value, and a catalyst purification treatment method is required for secondary treatment of exhaust gas. It is well-known that the secondary combustion of the exhausted gas outside the cylinder under a catalytic effect will generate a high temperature, and has a great influence on the service life and power performance of engines; the catalyst has a short service life, and the conversion efficiency will step down during use; and the use of a catalytic converter will limit the area of an exhaust passage and thus increase the flow resistance of the exhaust.
At present, the traditional cylinder scavenging duct is integrally arranged on one side of the cylinder, that is, the scavenging port and the scavenging duct inlet are on the same side of a cylinder wall. The scavenging duct is generally arranged as a vertical passage or forms an oblique passage at a small angle relative to a cylinder mounting surface, is limited by the cylinder height, the position of the scavenging port and the method for mounting the cylinder, the length of the scavenging duct is relatively short, the closed middle passage has a small volume, and then the total amount of air stored in the closed passage of the scavenging duct is also reduced. Air stored in the scavenging duct firstly performs scavenging in an initial scavenging stage, and then the flue gas mixture follows up for scavenging. In a forced scavenging stage in an exhaust gas process of an engine, three types of gas, i.e., exhaust gas, gas mixture and air, are present in the combustion chamber at the same time, the layer of the exhaust gas and the gas mixture being spaced by air. Due to the flowing of gas and the irregular space in the combustion chamber, the air spacing layer is thin, some of the gas mixture and the exhaust gas are in contact to present mutual permeation, and a very small amount of the gas mixture will be discharged along with the exhaust gas at the end of exhausting stage.
In addition, when air is stored in the closed passage of the scavenging duct, the scavenging duct inlet is an interface of the air and the gas mixture. It is known that in the period from the air being stored in the closed passage until the air exits the closed passage in the cylinder, the air is in full contact with the gas mixture to present mutual permeation, and the larger the area at the scavenging duct inlet, the more the gas mixture and the air mutually permeated; and the greater the ratio of the area to the length of the scavenging duct inlet, the higher the ratio of the gas mixture mutually permeated to the air. The permeation will lower the content of pure air in the combustion chamber, reduce the thickness of the air layer, decrease the spacing effect of the air on the gas mixture and the exhaust gas, and increase the possibility of the gas mixture escaping through the exhaust port.